I read a little about sleep/wakeup of DaVinci ARM part and compared it
with what I know from OMAP.
Now I wonder if DaVinci ARM can exit ARM wake-for-interrupt mode (WFI)
if ARM I- and F-IRQs are disabled?
sprue14a [1] states (section 8.4.1, page 92):
-- cut --
The following sequence describes the procedure to wake up from the
wait-for-interrupt mode:
- To wake up from the wait-for-interrupt mode, trigger any enabled
interrupt (for example, an external interrupt).
- The ARM’s PC jumps to the IRQ vector and you must handle the
interrupt in an interrupt service routine (ISR).
Exit the ISR and continue normal program execution starting from the
instruction immediately following the instruction that enabled
wait-for-interrupt mode: mcr p15, #0, r3, c7, c0, #4.
-- cut --
Assuming that SDRAM is in self refresh mode and therefore WFI
enter/exit code is in internal (TCM) SRAM, ISR has to be in SRAM as
well? This implies that
- ARMs IRQ entry has to be mapped by MMU from standard Linux location
to SRAM
- All MMU translation for ARMs IRQ/ISR to internal SRAM has to be in
TLB cause MMU tables in SDRAM are not accessible
- ISR can't really handle data cause all (Linux-) data handling code
is located in SDRAM (still in self refresh). It has to be a "minimal"
ISR, main job is to wake the system and not to handle data.
What I know from OMAP is that ARM is able to exit WFI with ARM I- and
F-IRQ *disabled* (local_irq_disable() & local_fiq_disable()). On OMAP,
while going to sleep, still running from SDRAM, you configure some
interrupts as wake sources and then disable ARM I- and F-IRQ. Then you
jump to SRAM, enable SDRAM self refresh and go to WFI. Then, ARM can
still wake from interrupt, but *doesn't* jump to ISR immediately cause
I- and F-IRQ are still off. Enabling SDRAM, jumping back to SDRAM,
enabling ARMs I- and F-IRQ and *then* system jump and use normal Linux
ISRs for wake interrupt.
This is quite elegant and avoids complex programming steps like above
necessary on DaVinci? Or did I miss anything here?
Best regards
Dirk
[1]
http://focus.ti.com/dsp/docs/dspsupporttechdocsc.tsp?sectionId=3&tabId=409&familyId=1302&abstractName=sprue14a
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